Pass by reference

Hi, I'm having a problem using pass by reference to pass arrays and floats through functions. This is my assignment:
http://www.comp.dit.ie/mcollins/ft22...signment2.html

The code I've got so far (which is pass by value as its all i can do):

Code:

#include <stdio.h>
#define SIZE 5

main()
{
    int i;
    float temps[SIZE];

    printf("\nPlease enter %d float numbers: \n\n", SIZE);
    
    read(temps);
    display(temps);
    
}            

read(float temps[])
{
    int i;
    
    for (i = 0; i < SIZE; i++)
    {
        scanf("%f", &temps[i]);
    }
}

display(float temps[])
{
    int i;
    
    for (i = 0; i < SIZE; i++)
    {
        printf("%f ", temps[i]);
    }
}

How do I change this to pass by reference?

Many thanks in advance.

well your assignment says

You must use pointer notation to access array elements - NOT subscripts

here is some reading on pointersand arrays
http://www.cprogramming.com/tutorial/lesson6.html
http://www.cprogramming.com/tutorial/lesson8.html

>How do I change this to pass by reference?
C only supports pass-by-value. However, you can fake it with pointers:

Code:

#include <stdio.h>

/* Pass by value */
void f0(int x)
{
  ++x;
}

/* Pass a pointer by value, simulate pass-by-reference */
void f1(int *x)
{
  ++*x;
}

int main(void)
{
  int a = 0;
  int b = 0;

  f0(a);
  f1(&b);
  printf("a = %d\n", a);
  printf("b = %d\n", b);

  return 0;
}

The interesting thing is that arrays are passed as a pointer to the first element by default. These two function declarations are equivalent:

Code:

void f0(int array[]);
void f1(int *array);

So technically, you are already passing by "reference"[1].

[1] The quotes are important.

Thanks a lot for the replies. I've written the code but there are a couple of bits I'm finding it hard to do. Here's the code:

Code:

#include <stdio.h>
#define SIZE 5

void read (float *, float);
void display (float *, float);
void sort (float *, float);
void conv_far (float *, float);
void conv_cel (float *, float);

main ()
{
    float values[SIZE];    //declaring variables and initalizing them
    int i;
    int option = 0;

    while (option != 6) //loop
    {   //start loop
        printf("Please select an option from the list below:\n"); 
        printf("1. Enter values\n");
        printf("2. Display values\n");            //displays menu
        printf("3. Sort temperatures in increasing order\n");
        printf("4. Convert temperatures to Fahrenheit\n");
        printf("5. Convert temperatures to Celsius\n");
        printf("6. Exit Program\n");
        
        scanf("%d", &option);                    //allows user to choose option
        fflush (stdin);
        
        switch (option)                //switch statement, chooses option selected by user and performs task
        { // start switch
                case 1:
                {
                    read (values, SIZE);
                    fflush (stdin);
                    break;
                }
                
                case 2:
                {
                    display (values, SIZE);
                    break;
                }    
                
                case 3:
                {
                    sort (values, SIZE);
                    break;
                }
                
                case 4:
                {
                    conv_far (values, SIZE);
                    break;
                }
                
                case 5:
                {
                    conv_cel (values, SIZE);
                    break;
                }
                
                case 6:
                {
                    printf ("program ended\n");
                    break;
                }
                
                default:
                {
                    printf ("\nerror! Please enter a number between 1-6\n\n");
                }
        } //end switch

    } //end loop

}

void read (float *array, float no_of_els) // function for case 1
{
   int i = 0;

   for (i = 0; i < no_of_els; i++)
   {
       printf ("enter temperature %d: ",i+1);
       scanf  ("%f", &(*(array+i)));
   }
   printf("\n");

}

void display (float *array, float no_of_els)  // case 2
{
    int i = 0;

     for (i = 0; i < no_of_els; i++)
     {
         printf ("temperature %d is %4.2f\n",i+1,*(array+i));
     }
     printf("\n");
}

void sort (float *array, float no_of_els) // case 3
{
    int i, item, temp;
    
    for (item = 0; item < no_of_els - 1; item++)
    {
        for (i = item + 1; i < no_of_els; i++)
        {
            if (*(array+i) < *(array+item))
            {
                temp              = *(array+item);
                *(array+item) = *(array+i);
                *(array+i)    = temp;
            }
        }
    }
   
   for (i = 0; i < SIZE; i++)
   {
       printf ("%4.2f\n", *(array+i) );
   }
   printf("\n");   
}

void conv_far (float *array, float no_of_els) // case 4
{
    int i = 0;
    float celcius = 0;
    float fahrenheit = 0;
    
    for (i = 0; i < no_of_els; i++)
    {
        celcius = celcius + *(array+i);
        fahrenheit = fahrenheit +  (((celcius/5) * 9) + 32);
        printf ("temperature %d is %4.2f\n", i+1, fahrenheit);
        fahrenheit = 0;
        celcius = 0;
    }
    printf("\n");
}

void conv_cel (float *array, float no_of_els) // case 5
{
    int i = 0;
    float celcius = 0;
    float fahrenheit = 0;
    
    for (i = 0; i < no_of_els; i++)
    {
        fahrenheit = fahrenheit + *(array+i);
        celcius = celcius +  (((fahrenheit-32) / 9) * 5);
        printf ("temperature %d is %4.2f\n", i+1, celcius);
        fahrenheit = 0;
        celcius = 0;
    }
    printf("\n");
}

The bits I'm finding it hard to do are:

5. Provide an option to allow a user to convert the temperatures back to Celsius (only if the temperatures have already been converted to Fahrenheit). NB: You must find a way that will prevent the user selecting this option if the temperatures are already in Celsius.

and

The user should only be allowed to select option 4 (convert to Fahrenheit) if the contents of the 1-D array are in Celsius. Appropriate error messages should handle any potential problem. The same should happen if the user selects option 5 (convert to Celsius) i.e. this should only be allowed if the contents of the 1-D array are in Fahrenheit.

thanks again.

and just a quick question. When writing the pseudo code what should I do for the new functions?

Cheers.

Does anyone have any idea? BTW is this what the pseudo code should look like?:

Code:

BEGIN MAIN
   
   WHILE (option != 5) 
      DISPLAY Menu
      INPUT Option
      
      SWITCH country
        
	CASE 1
	  FUNCTION Read
	BREAK
	
	CASE 2
	  FUNCTION Display
	BREAK
	
	CASE 3
	  FUNCTION sort
	BREAK

	CASE 4
	  FUNCTION Farenheit
	BREAK

	CASE 5
	  FUNCTION Celsius
	BREAK
	
	CASE 6
	  EXIT PROGRAM
	BREAK
   
   END WHILE

END MAIN

START Read
  
   FOR

You just need a flag in main() to tell you whether the temperatures are currently in Celsius or Fahrenheit.

Code:

int celsius_mode;

If when you enter the temperatures, they are in Celsius, when you return from the read() function, set this to true(1):

Code:

celsius_mode = 1;

Whenever you convert to Fahenheit, set this to false(0).

Code:

celsius_mode = 0;

Now you know which mode you are in at any time, so if you need to check whether you're already in Celsius, you can do:

Code:

if (celsius_mode)
or
if (celsius_mode == 1)

Or conversely, to check whether you're in Fahrenheit:

Code:

if (!celsius_mode)
or
if (celsius_mode == 0)

thanks a lot, thats a great help.

I've noticed that when I convert the temperatures to Fahrenheit and then try to convert back to Celsius, all its doing is using the Celsius formula ((Celsius = ( ( (Fahrenheit - 32) / 9) * 5 )) on the original Celsius values. How do I get it to convert the Farenheit values back to Celsius?

Thanks.

>How do I get it to convert the Farenheit values back to Celsius?
Well, for example take conv_far():
> celcius = celcius + *(array+i);
> fahrenheit = fahrenheit + (((celcius/5) * 9) + 32);

Here you convert to fahrenheit, but never store the value back into the array. So you would add this:

Code:

*(array+i) = fahrenheit;

Or you could do the whole conversion in one statement vs three by:
*(array+i) = *(array+i) / 5. * 9. + 32.

Also, it makes more sense to write what you have as:

Code:

        celcius = *(array+i);
        fahrenheit = (((celcius/5) * 9) + 32);
        *(array+i) = fahrenheit;

As, there's no need for the addition in there, or initalizing celcius and fahrenheit to 0.

Now you also need to do the same for your conv_cel() function.

You're a star snoopy.

> fflush (stdin);
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